Delay diagnosing asm constraints that require immediates until after inlining

Summary:
An inline asm call may result in an immediate input value after inlining.
Therefore, don't emit a diagnostic here if the input isn't an immediate.

Reviewers: joerg, eli.friedman, rsmith

Subscribers: asb, rbar, johnrusso, simoncook, apazos, sabuasal, niosHD, jrtc27, zzheng, edward-jones, rogfer01, MartinMosbeck, brucehoult, the_o, PkmX, jocewei, s.egerton, krytarowski, mgorny, riccibruno, eraman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60943

llvm-svn: 368104